1. Field of the Invention
The present invention relates to a premixed air-fuel mixture supply device for supplying a premixed air-fuel mixture to a combustor for a gas turbine or an aircraft engine and, more particularly, to a premixed air-fuel mixture supply device capable of atomizing fuel satisfactorily while the associated combustor is in a low-load operation.
2. Description of the Related Art
A conventional combustor for a gas turbine or an aircraft engine has a combustor casing, and a cylindrical or annular combustor liner disposed in the combustor casing to define a combustion chamber. A fuel nozzle is connected to a head part of the combustor liner. The combustor casing and the combustor liner define an air passage through which air supplied by an air compressor flows into the combustion chamber.
When fuel is injected in air for diffusive combustion in the combustion chamber of this combustor of a gas turbine or an aircraft engine, high-temperature regions are formed locally in the combustion gas, and the high-temperature regions increases the concentration of NOx in the combustion gas.
Interest in environmental problems has progressively increased in recent years and restrictions on environmental condition have been intensified. The inlet temperature of recent gas turbines and aircraft engines, namely, the outlet temperature of the combustors of gas turbines and aircraft engines, has been raised to improve the thermal efficiency of the gas turbines and aircraft engines. However, the local high-temperature regions in the combustion gas produced by diffusive combustion increase and the concentration of NOx increases accordingly as the outlet temperature of the combustors of gas turbines and such increases. Therefore, measures for reducing NOx is very important.
A gas turbine combustor with a lean premixed, prevaporized combustion system (a prevaporized, premixed air-fuel mixture lean-burn type gas turbine combustor for a gas turbine) is proposed to reduce the concentration of NOx in the combustion gas. In this gas turbine combustor, part of fuel is supplied as pilot fuel into a pilot combustion region in a combustion chamber to produce high-temperature combustion gas by stable combustion, and a lean air-fuel mixture is burned in a main combustion region around and below the pilot combustion region for lean-burn combustion that scarcely produces NOx. When a liquid fuel is used, the liquid fuel is vaporized beforehand to produce a prevaporized, premixed air-fuel mixture for lean burn. An air-blast atomization type premixed air-fuel mixture supply device injects main fuel substantially perpendicularly to the flowing direction of combustion air.
A conventional combustor for a gas turbine or an aircraft engine has a combustor casing and a cylindrical or annular combustor liner disposed in the combustor casing to define a combustion chamber. A fuel nozzle for injecting fuel into the combustion chamber is disposed at one end of the combustor liner. The premixed air-fuel mixture supply device according to the present invention has a main fuel injecting unit and a prevaporizing, premixing unit for vaporizing and mixing the fuel injected by the main fuel injecting unit in addition to the pilot fuel injecting unit.
Referring to FIG. 5 showing a conventional combustor, compressed air supplied by an air compressor, not shown, flows through a space between a combustor casing 1 and a combustor liner 2. When the combustor is a forward flow combustor, air flows in the direction of the blank arrow (→) and the right end, namely, the downstream end, of the combustor casing 1 is closed. When the combustor is a backward flow combustor, air flows in the direction of the arrow (←), and the left end, namely, the downstream end, of the combustor casing 1 is closed. Combustion air reached the combustor head flows into a pilot combustion air passage 3 and a main combustion air passage 4. Although the main combustion air passage 4 shown in FIG. 5 is divided into two air passages 4a and 4b, the main combustion air passage 4 does not necessarily need to be divided.
Referring to FIGS. 6 and 7 showing a prevaporizing, premixing unit, pilot fuel is injected out through fuel injection holes 5a formed in a pilot fuel injection nozzle 5 and arranged at angular intervals. Swirl devices 6a and 6b for swirling combustion air are disposed above the fuel injection holes 5a. Main fuel is injected out through main fuel injection holes 7 arranged at angular intervals. Swirl devices 8a and 8b for swirling combustion air are disposed above the main fuel injection holes 7. An atomization lip 9 extends downstream from the swirl devices 8a and 8b to atomize the main fuel. A prevaporizing, premixing chamber 10 is formed below the atomization lip 9. A premixed air-fuel mixture produced in the prevaporizing, premixing chamber 10 is supplied into a combustion chamber 15 below the prevaporizing, premixing unit. The premixed air-fuel mixture burns in the combustion chamber 15. In FIG. 7, the premixed air-fuel mixture supply device is provided with a single swirling device 8 instead of the two swirling devices 8a and 8b shown in FIG. 6, and is not provided with any member corresponding to the atomization lip 9
Related techniques are disclosed in JP-A 8-42851, JP-A 9-145057 AND JP-A 2002-206744.
A fuel injector included in combustor for a gas turbine or an aircraft engine operates according to load on the combustor that varies in a wide load range Therefore, in most cases where the combustor is in a low-load operation requiring fuel injection at a low rate, fuel injected by the fuel injector is unable to flow across air currents to or near to a wall surrounding the fuel injection valve and unable to vaporize satisfactorily. The fuel may be able to flow across air currents to or near to the wall when the number of the fuel-injecting hole is reduced or the diameter of the fuel injecting holes is reduced. If such measures are taken, the fuel needs to be fed at a very high fuel feed pressure when the combustor is in a high-load operation, and a large fuel feed system is necessary. The reduction of the number of fuel injecting holes deteriorates the mixing of fuel and combustion air; The fuel injecting holes may be closed by caulking when the diameter of the fuel injecting holes is reduced.